A relay device is a device that is located between a base station and a terminal to receive and amplify signals from the base station and the terminal. That is, it refers to a device that receives and amplifies a signal from the base station to transmit the amplified signal to the terminal, and receives and amplifies a signal from the terminal to transmit the amplified signal to the base station. The relay device is installed in the place where a wireless transmission and reception condition between the terminal and the base station is bad, to relay a signal and is mostly installed under or in the building where it is difficult for a signal from the base station to arrive at. The structure of the relay device varies depending on a building structure or the characteristic of a shadow area, and when especially used in the building, a distribution/relay device which uses several distributed antennas is used.
The distribution/relay device may match and relay an outdoor base station outside the building or match and use a base station for only the distribution/relay device, according to the capacity of a user connected to the distribution/relay device. In the case of Long Term Evolution (LTE), each building matches and uses a small base station, i.e., femtocell because the capacity of data occupied by a user is large.
The distribution/relay device is connected to several distributed antennas by using a coaxial cable. The coaxial cable has a characteristic that it is possible to transmit a high-frequency signal used in a base station without a change in frequency as it is. However, since installing the coaxial cable inside the building costs high, a distribution/relay device that uses an UTP line, one of LAN cables has been studied.
As an example, Korean Patent Publication No. 2010-0068688 employs a method that includes converting a high-frequency signal received from a base station or communication device by a wired or wireless connection into a digital signal, and transmitting, by a digital signal processing unit, the digital signal to an Ethernet cable by using time division multiplexing. The technique connects a signal distribution device to a plurality of remote signal transmission and reception devices by an UTP line, a cheap communication cable that has been previously installed without installing a separate cable and relates to a relay device that may transmit an existing Internet signal and a mobile communication signal through the UTP cable simultaneously.
However, it is still difficult to transmit two or more femtocells to the UTP cable due to the limit of a data transmission capacity of an UTP transmission line. Thus, there is a need for a study on transmitting two or more femtocell signals to a single UTP line.